User guide

Internetworking Primer A-11
LANs and WANs
For example, the IEEE standard 802.3 (Ethernet) defines the physical layer (connectors,
voltages, and binary logic to enable communication) as well as the data link layer (framing
data structures, addressing packets for local delivery, and error checking).
Once the physical and data link layers are installed in a network and working correctly, the
network layer and its associated devices can use these services to route packets efficiently
across the room or around the world.
Building a network involves understanding the OSI reference model in each portion of the
network. Sketching the model out on paper is a great help in designing, administering, and
troubleshooting networks. (See Figure A-2.)
Figure A-2 The OSI Reference Model
LAN Technologies
LANs operate at Layer 1 (the physical layer) and Layer 2 (the data link layer) and are
usually used to connect computing devices within a single building. For example, a LAN
in a small office may include three PCs, a print server, and a file server.
The three most common physical LAN implementations are Ethernet, Token Ring, and
Fiber Distributed Data Interface (FDDI). These implementations have been standardized by
Institute of Electrical and Electronic Engineers (IEEE) and the American National
Standards Institute (ANSI).
H3801
Physical layer
Data link layer
Network layer
Transport layer
Presentation
layer
Application
layer
Session layer
Physical layer
Data link layer
Network layer
Transport layer
Presentation
layer
Application
layer
Session layer